The invention relates generally to the stabilization of suspensions of solids in fluids and in particular to the suspension of pulverized coal particles in heavy fuel oils.
At the present time, many users of fuel oils are faced with the prospect of converting to the use of coal, in part because of the increasing cost of fuel oils and in part to avoid dependence upon liquid fuels which may be in short supply in the future. However, conversion from the use of fuel oils to coal requires substantial investment in costly equipment.
One alternative to the installation of coal-firing equipment is the firing of coal-oil mixtures. It has been recognized that such mixtures may be fired in conventional oil-firing equipment and thereby gaining the advantages of using coal while minimizing investment in new equipment. However, in order for such coal-oil mixtures to be fired successfully, it is essential that a uniform mixture be supplied to the oil burners. One possible approach would be to mix the coal in a pulverized form with fuel oil immediately before firing in order to provide a uniform mixture. Such a procedure is difficult to carry out accurately, and it would be preferable to premix the coal and oil in large quantities and then to supply the oil burners via the conventional oil supply equipment. In order to accomplish such an objective, it is necessary to prevent the settling of coal particles. It is theoretically feasible to divide the coal so finely that when suspended in fuel oil, essentially no settling would occur. However, such a procedure would require the pulverizing of the coal to form particles of near submicron size or smaller. The substantial power input needed as well as the large investment in equipment for size reduction of the coal would offset the savings to be obtained from substituting coal for a portion of the fuel oil being used. It would be preferred to have a method for stabilizing coal-in-oil mixtures in which the coal particles are of a size which will ordinarily settle in a short time unless prevented from settling, i.e., the particles produced by conventional pulverizing equipment, for which the technology and operating parameters are well known and easily utilized.
Suspension of coal particles in liquids has been the subject of a number of U.S. patents. Adams in U.S. Pat. No. 1,939,587 disclosed the use of coal-in-oil suspensions for use as boiler fuel, showing that it was possible to stabilize coal particles ground to less than 200 mesh (Tyler) in oil which contained at least 6% "fixed carbon" (as defined by Adams). Hansley in U.S. Pat. No. 2,668,757 was concerned with dispersing carbon particles generally in organic liquids and disclosed a method of dispersing particles of 1-200 microns in size by forming an alkali metal soap using a fatty acid and an alkali metal such as sodium. Other patents have been concerned with the dispersion of coal in water to facilitate the transportation of coal in the form of water slurries. Examples of such patents are U.S. Pat. No. 3,941,552 and U.S. Pat. No. 3,524,682.
A related subject of considerable commercial interest is the dispersion of sludge particles in the lube oils employed in internal combustion engines. Numerous U.S. patents are concerned with such dispersants, including the following:
U.S. Pat. Nos. 3,172,892; 3,341,542; 3,184,474; 3,347,645; 3,194,812; 3,381,673; 3,194,814; 3,381,022; 3,219,666; 3,594,497; 3,272,746; 3,658,495; 3,340,281.
LeSuer in U.S. Pat. No. 3,172,892 discloses lube oil dispersants which are derived from the reaction of a substituted succinic acid or anhydride with ethylene polyamines. In general it was found that the dispersant effect was related to the size of the molecule produced by the chemical reaction. It is typical of the teachings of LeSuer and others in this field that the dispersants were to be produced by the chemical reaction of acids or anhydrides with amines. Such reactions produce water as a by-product, which is removed before the dispersant is ready for use in lube oils. While such dispersants might be expected to be useful for suspension of coal particles in oil, as will be shown hereinafter, they have been found to be generally ineffective. They are typically used under the dynamic conditions existing in an operating engine when dispersing particles in oil and as will be shown, such dispersants actually have a detrimental effect under the long term static conditions which pertain to the suspension of coal in oil.
The present invention has as one objective the provision of a method for dispersing coal particles at least 70-80% of which are smaller than 200 mesh in fuel oils whereby they may be held in storage for extended periods before being burned.
Another aspect of the invention has as its object a composition of matter comprising a suspension of coal particles at least 70-80% of which are smaller than 200 mesh size in fuel oils which can be held in suspension with minimal settling for periods of up to 60 days.
In still another aspect of the invention, a method of stabilizing coal-in-oil suspensions at low cost is provided which permits firing of coal-in-oil suspensions and taking advantage of the inherent economics of such firing.